34 | New Scientist | 26 September 2020
of 87.7 years, which is why it was chosen as the
power source for Voyager 2. The plutonium
dribbles out a stream of alpha particles,
generating heat that is turned into electricity
by the probe’s three roughly suitcase-sized
radioisotope thermoelectric generators.
Radioactivity has a bad reputation, but
not all types are equal. Gamma radiation
penetrates human tissue most deeply and
is dangerous. Beta radiation isn’t so bad.
Alpha radiation doesn’t get through the skin,
so it is only damaging if it gets loose inside
you. In fact, pacemakers were powered with
well-contained radionuclide thermoelectric
generators until the early 1970s.
The concept that the US Army is eyeing up
is a kind of nuclear power that blends some
of the best bits of the other types – it could be
powerful, safe and long-lasting. It depends
on the fact that the protons and neutrons of a
particular element can be clustered together
in different arrangements in an atomic
nucleus. These are called isomers and each
has a different energy. Atoms usually reside
in what is normally their most stable isomer,
the ground state. Higher energy isomers
tend to quickly rearrange themselves back
to this state. But there are a few high-energy
isomers that hang around for a long time.Pent-up energy
In 1998, Carl Collins at the University of Texas
used a particle accelerator to prepare one
of these stable high-energy isomers, called
hafnium-178m2 (the m2 notation means
this is the second isomer of hafnium-178).
He then fired X-rays at its nucleus and
claimed that this shifted the nucleus to its
ground state, releasing a burst of gamma
rays. These would be hard to tap as an energy
source because they are so dangerous, but
Collins saw it as proof of principle that
nuclear isomers could be useful power
sources. He thought they could even be
used as a new type of nuclear bomb.
Many scientists ridiculed Collins’s claims,
arguing that he had to put in more energy to
trigger the isomer shift than he got out. Plus,
the fact that you need a particle acceleratorto make the hafnium isomer meant it could
only be produced in small quantities at great
expense. The episode became known as the
“hafnium controversy”.
Other high-energy isomers might get
around the problems. For example,
tantalum-180m occurs naturally, if rarely,
in mineable tantalum deposits. Silver-108m
produces beta radiation, which is less
dangerous and easier to tap. None of this
makes isomer power a safe bet, but the
pay-off from creating an effectively unlimited
energy source may make it worthwhile. A
similar rationale applies to the £11.6 billion
being spent on the ITER fusion reactor in
France, even though it is intended merely
as a technology demonstration and won’t
generate power.
Collins’s approach was to get all the
pent-up power of an isomer out in one
go. But there is, in principle, a different
and arguably more useful method. We have
known about it for decades, it just hasn’t
been properly pursued.
Imagine you have a lump of radioactive
isomer that, like hafnium-178m2, is highNASA
/JP
L-CAL
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CHenergy but stable. You could have this sitting
safely in a container for a long time because
it emits barely any radiation. When you need
some power, you convert a small amount
of it into its ground state, which is less stable
and begins to radioactively decay quickly.
This gives you a generator akin to the one
in Voyager 2, but which can be cranked up
in power at will.
James Carroll at the US Army Research
Laboratory in Adelphi, Maryland, has been
investigating whether interconverting
isomers in this way is possible. One potential
way to do it, first proposed in 1976, involves
firing an electron at an isomer and it being
absorbed into an orbit around the nucleus.
This prompts the protons and neutrons to
rearrange. It is called nuclear excitation by
electron capture (NEEC).
Carroll and his team used a particle
accelerator at Argonne National Laboratory
near Chicago to create a beam of
molybdenum-93m atoms, with a half-life
of about 7 hours. This beam was travelling
at about 10 per cent of the speed of light,
fast enough to strip away some of theVoyager 2 was
launched more than
40 years ago, yet its
battery is still going